Tunneling vs gen rushing
This needs to be explained so killer mains can understand.
Why actual tunneling and gen rushing are different.
Gen rushing=survivors enter map and all immediately leap on generators (or if they came in together they jump on one.)
The killer player is still playing freely the entire time. If the survivors are together then he can still use perks or guard specific gens for a 3 gen strat. Gen time together was also nerfed.
If survivors are all on 4 separate gens he can go to any of multiple gens, and attack someone. Still playing freely. He can attack, lob bottles, guard gens, etc.
Tunneling: when a killer main specifically dedicates himself to killing a specific survivor fast as possible. Hooking that person fast as possible, guarding and/or downing them again as fast as possible. The survivor is not getting to play freely, he is spending his time on the hook and likely getting down seconds after being unhooked, then being hooked again for the most limited, restricted, and miserable experience possible. Survivors don't exactly have flamethrowers or pocketknives.
For a simple comparison:imagine playing Star Wars Battlefront. If your team is losing badly you can still give it your best shot. Flank the enemy, surprise attacks, etc. You can still play freely and make a valiant last stand, jump in a tank, put a turret down and duck and shoot from cover, etc.
Now imagine playing Star Wars Battlefront and instead of your team losing badly, an invinceable player dedicates themselves to killing you as soon as you spawn and your gameplay consists of running away then dying quickly. The other people on your "team" might win, but you're not playing freely, the game might as well a downgraded phone edition without half the game. Who on earth in their right mind would spend their free time on that?